Photoinduced Isomerization-Driven Structural Transformation between Decanuclear and Octadecanuclear Gold(I) Sulfido Clusters

Article abstract of DOI:10.1021/jacs.5b01676

Upon photoirradiation, isomerization of the ligands, 1,2-bis(diphenylphosphino)ethene (dppee) from the cis to the trans form in polynuclear gold(I) sulfido clusters, led to the structural transformation of the decagold(I) cluster to the octadecagold(I) cl

Full text of DOI:10.1021/jacs.5b01676

Communication
pubs.acs.org/JACS
Photoinduced Isomerization-Driven Structural Transformation
Between Decanuclear and Octadecanuclear Gold(I) Sulfido Clusters
Liao-Yuan Yao and Vivian Wing-Wah Yam*
Institute of Molecular Functional Materials and Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong
Kong, P. R. China
S
* Supporting Information
edge, similar photoisomerization properties have never been
applied to the polynuclear gold(I) cluster system.
Here we report the unprecedented photoinduced isomer-
ization-driven structural reconfiguration of polynuclear gold(I)
complexes. The chlorogold(I) precursors, cis-[dppee(AuCl)2]
and trans-[dppee(AuCl)2] (Chart 1),10,11 have been employed to
ABSTRACT: Upon photoirradiation, isomerization of the
ligands, 1,2-bis(diphenylphosphino)ethene (dppee) from
the cis to the trans form in polynuclear gold(I) sulfido
clusters, led to the structural transformation of the
decagold(I) cluster to the octadecagold(I) cluster. Both
polynuclear μ3-sulfido gold(I) clusters have been fully
characterized by NMR, mass spectrometry, elemental
analysis, and single crystal X-ray diffraction analysis. The
transformation process could be readily detected and
monitored by UV−vis absorption, emission, and 31P NMR
spectroscopy in solution. Supported and driven by Au(I)···
Au(I) bonding interactions, the nuclearity and symmetry
of these clusters were largely different from each other,
resulting in completely distinct photophysical features.
Chart 1. Chemical Structures of the cis- and trans-dppee Based
Chlorogold(I) Precursors, cis-[dppee(AuCl)2], and trans-
[dppee(AuCl)2]
urophilic interaction,1 with the interaction energy com-
assemble luminescent polynuclear gold(I) μ3-sulfido clusters, 1
(Au10) and 2 (Au18), respectively. Interestingly, the decagold(I)
cluster could be quantitatively transformed to octadecagold(I)
clusters in solution, mediated by the transformation of the
organic ligands from cis- to trans-dppee, which could be
monitored by 31P{1H} NMR, UV−vis absorption, and emission
spectroscopy. This work shows promises to the uncovering of the
nature and the mechanism for the formation of polynuclear
gold(I) clusters.
Reaction of cis-[dppee(AuCl)2] with H2S led to a clear yellow
solution, which after evaporation afforded a yellow solid. Yellow
block crystals of 1 could be obtained by recrystallization of the
yellow solid from a dichloromethane−methanol−diethyl ether
system. The 31P{1H} NMR spectrum of the yellow crystals in
CD3OD shows a pair of doublets of a two-spin AX pattern at δ =
13.7 and 17.0 ppm (JP−P = 56.8 Hz), indicating two different
phosphorus environments. The molecular ion cluster observed at
m/z 1841.5 in the MALDI-TOF and ESI mass spectra is
attributed to [Au10(μ-cis-dppee)4(μ3-S)4]2+ ([1]2+) (Figure S1
and S2). The formation of cluster 1 has been further confirmed
by single-crystal X-ray diffraction analysis. The chloride salt
[1]Cl2 crystallizes in orthorhombic space group Pna21. The ten
gold(I) atoms are bridged by four μ3-S atoms and surrounded by
four diphosphine ligands (Figure S3). The cis forms of the dppee
ligands are unambiguously observed in the cluster structure, with
two P atoms and two bridging C atoms all on the same plane. The
Au···Au bond distances are in the range of 2.88−3.16 Å,
2
Aparable to that of hydrogen bond, has been widely utilized
to construct diverse architectures.2,3 Perturbed by gold(I) and
gold(I)···gold(I) interactions, the energy gap between the
HOMO and the LUMO of these assemblies could be largely
narrowed, resulting in promising luminescent properties.2,3 In
the past decades, there has been an increasing research interest in
the construction of polynuclear gold(I) aggregates via gold(I)···
gold(I) interactions.2−8 Polynuclear sulfido gold(I) complexes,
as one of the most stable systems, are ubiquitous in the gold
family.2a,3,6,7,8a−f Much attention has been focused on the
development of polynuclear gold(I) complexes with rich
luminescence properties in recent years,6−8 especially the
polynuclear μ3-sulfido gold(I) clusters bridged by bi- or
tridentate phosphine ligands,8a−f via metal···metal interaction
directed self-assembly.
On the other hand, cis−trans isomerization can frequently be
observed in CC,9a,b CN,9c,d and NN9e,f bond containing
derivatives, which have been widely used to design and
synthesize functional molecular switches.9 Generally, the trans
forms of these double bond based compounds are thermody-
namically more stable than the cis forms.9,10 Bruce and co-
workers have reported that the trans isomer of the uncoordinated
1,2-bis(diphenylphosphino)ethene (dppee) ligand is 23 kJ mol−1
more stable than cis-dppee.10a Nevertheless, the free cis-dppee
could not undergo isomerization upon photoirradiation.10a,c By
coordination to gold(I), several dinuclear gold(I) cis-dppee
complexes have been demonstrated to possess photochemical
isomerization reactivity.10 However, to the best of our knowl-
Received: February 18, 2015
Published: March 5, 2015
© 2015 American Chemical Society
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DOI: 10.1021/jacs.5b01676
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Communication
suggesting the presence of aurophilic interactions (Table S1).
Single crystal structure determination indicates no symmetry in
the solid state. However, similar to our previously reported Au10
clusters,8b,e,f only two nonexchangeable P environments are
observed in the 31P{1H} NMR spectrum, indicating the local S4
symmetry of cluster 1 in solution (Figure S4). The fluxionality of
the cluster in solution might be responsible for the symmetry
increase.
of several organic ligands and gold atoms (Figure S9) establishes
the formation of an octadecanuclear gold(I) cluster. The high
resolution molecular peak observed at m/z 3089.4 in the ESI
mass spectrum is in good agreement with the simulated isotopic
distribution of [Au18(μ-trans-dppee)6(μ3-S)8]2+ (Figure S10).
These two clusters have been further characterized by 1H NMR
spectroscopy and elemental analysis (see Supporting Informa-
tion, experimental section).
On going from the cis to the trans products in cis-
[dppee(AuCl)2] and trans-[dppee(AuCl)2], a blue shift in the
UV−vis absorption spectra was observed. This has been
attributed to aurophilic interactions that stabilize the first excited
singlet state of cis-[dppee(AuCl)2].10b Similar blue shifts could
also be found in other cis-to-trans-dppee-containing dinuclear
gold(I) complexes.10 On the contrary, in the polynuclear gold(I)
cluster system, a cis-to-trans conversion from 1 to 2 results in a
dramatic red shift in electronic absorption spectra. The UV−vis
absorption spectra of 1 and 2 show several low-energy absorption
shoulders at ca. 320, 365, and 425 nm and at ca. 380 and 465 nm
(Figure 1), respectively, which are tentatively assigned to ligand-
Interestingly, on exposure to ambient light irradiation for
several days, the solution color of [1]Cl2 changes slowly from
bright yellow to orange-red (Figure S5). A red crystalline solid
([2]Cl2) has been isolated from the orange-red solution. By
exchanging the counter-anions of the red complex from Cl− to
−
PF6 by metathesis reaction, red needle-shaped crystals of
[2](PF6)2 suitable for single crystal X-ray diffraction analysis
have been obtained and determined. The identity of the cluster
cation was established by single crystal X-ray structure
determination as [Au18(μ-trans-dppee)6(μ3-S)8]2+ ([2]2+), in
which cis-dppee was transformed to trans-dppee via photo-
chemical isomerization. [2](PF6)2 crystallizes in the monoclinic
C2/c space group, and the cluster cation 2 consists of 18 gold(I)
atoms, eight μ3-bridging S atoms, and six trans-dppee
diphosphine ligands (Figure S6). Similar to its related
[Au18Se8(dppe)6]2+ and [Au18S8(dppe)6]2+,5b,7g the gold skel-
eton of 2 consists of two Au9 macrocycles interlocked with each
other through aurophilic and Au−S interactions (Figure S7).
The cluster cation could also be viewed as one Au6S2 center
surrounded by two Au6S3(trans-dppee)3 crowns (Figure S8).
The Au···Au contacts are in the range of 2.95−3.17 Å (Table S2).
Relatively long Au···Au distances (about 3.6 Å) could be found
between the three gold(I) atoms (Au(7), Au(8), and Au(9))
coordinated to the same μ3-S (S(3)) center in each Au6S2 unit
(Figure S8).
On the other hand, starting from the chlorogold(I) precursor,
trans-[dppee(AuCl)2], cluster 2 could also be assembled by
reaction of the trans-[dppee(AuCl)2] with H2S in solution
(Scheme 1). 31P{1H} NMR analysis of [2]Cl2 shows only one
Figure 1. UV−vis absorption (left) and emission (right) spectra of
[1]Cl2 and [2]Cl2 in degassed methanol at ambient temperature.
Scheme 1. Self-Assembly Processes of Gold(I) Clusters and
Photoinduced Isomerization-Driven Transformations
to-metal charge transfer (LMCT; S → Au) transitions modified
by Au(I)···Au(I) interactions.8a−f The red shift might be
attributed to the increase in nuclearity from the Au10 to the
Au18 cluster that narrows the HOMO−LUMO energy gap. Their
luminescence properties are also found to behave differently.
Upon excitation at 450 nm, emission of cluster 1 both in solution
and in the solid state are weak with band maxima at around 630
nm, while cluster 2 emits much more intensely in the red region
both in solution and in the solid state at ca. 645 nm (Figures 1
and S11).
a
Under irradiation of a high intensity Xe lamp, structural
transformation of the cluster could be largely accelerated and
completed within several minutes. When the isomerization and
transformation were monitored by UV−vis absorption spectros-
copy in dilute methanol solution (5 × 10−5 M), the absorption
shoulder at around 450−550 nm was found to rise dramatically
(Figure 2), indicating the transformation of the clusters in
solution from 1 to 2. From the plot in Figure 2, the enhancement
of absorbance was found to slow down after 1 min, and finally the
transformation was completed within 2 min in dilute solution.
Meanwhile, with photoexcitation, the emission intensity would
increase significantly with emission maxima at around 645 nm,
corresponding to the formation of cluster 2 and suggesting the
transformation of the decanuclear to the octadecanuclear gold(I)
clusters (Figure S12).
a
Cluster cations 1 and 2 are represented by their crystal structures
(gray wire, C; orange ball, P; yellow ball, Au; red ball, S; protons and
counter-anions are omitted).
downfield-shifted singlet at δ = 39.8 ppm. On the basis of the
NMR spectrum and single crystal structure study, a local D3d
point group symmetry in solution could be assigned to cluster 2.
In the MALDI-TOF mass spectrum of cluster 2, the observed
fragments [Au17(μ-trans-dppee)3(μ3-S)8]+, [Au17(μ-trans-
dppee)2(μ3-S)8]+, and [Au13(μ-trans-dppee)3(μ3-S)6]+ with loss
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Figure 2. UV−vis spectral traces of [1]Cl2 at different times in dilute
methanol solution (5 × 10−5 M) under photoirradiation. The inset
shows the plot of absorbance at 465 nm at different irradiation time.
Figure 4. (a) Bridging models and (b) illustrations of the coordination
orientations of cis-dppee and trans-dppee in the solid state; selected
distances and bond lengths are shown. Other ligands, counter-anions,
and hydrogen atoms are omitted for clarity (gray, C; orange, P; yellow,
Au; red, S).
Further supporting evidence came from the 31P{1H} NMR
spectral changes (Figure 3). The initial 31P{1H} NMR signals of
irradiation, the photochemical isomerization of the diphosphine
ligand would disrupt the initial aurophilic interactions. These,
together with development of new gold(I)···gold(I) attractions,
have led to the configuration transformation of the cluster from
Au10 to Au18 units.
In summary, an unprecedented cluster transformation
involving a change in the cluster nuclearity has been mediated
by light. Upon photoirradiation, the decagold(I) cluster would
transform to an octadecagold(I) cluster in solution. UV−vis
absorption, emission, and 31P{1H} NMR spectroscopy,
combined with single crystal structural analysis, have been
employed to monitor the self-assembly processes and reveal the
transformation mechanism. Driven by cis-to-trans isomerization
of the organic diphosphine ligand dppee, the nuclearity and the
symmetry of the polynuclear sulfido gold(I) clusters have
switched significantly, resulting in different photophysical
behaviors. These properties endow polynuclear gold(I) sulfido
clusters 1 and 2 as attractive applications in novel photochromic
materials.
Figure 3. 31P{1H} NMR spectral changes in CD3OD during the
photoirradiation, with the initial concentration of [1]Cl2 at 10−3
M
(black, Au10; red, intermediate; blue, Au18).
1 appear at δ 13.7 and 17.0 ppm as a pair of doublets. After
photoirradiation with the Xe lamp for around 5 min, the yellow
solution turns to orange-yellow slightly, and the 31P{1H} NMR
spectrum indicates the appearance of a new singlet at δ 39.6 ppm
and a very broad signal at around δ 27 ppm, with the
disappearance of the signals of cluster 1. The singlet at ca. δ
39.6 ppm could be unambiguously assigned to [2]Cl2. The broad
signal indicates the existence of an intermediate12 before the
eventual assembly to octadecagold(I) cluster 2. Finally, after 20
min, only the signal of cluster 2 could be observed in solution,
indicating the completion of the photochemical isomerization
and cluster transformation processes.
The isomerization of the organic diphosphine ligands from cis-
to trans-dppee has resulted in the reconfiguration of the cluster
structures. In the single crystal structure of cluster 1, the distance
of the two P atoms in the cis-dppee ligand is 3.60 Å, with the short
Au···Au distance of 2.99 Å between two coordinated gold atoms
(Figure 4a). This indicates that the lone pairs on the two P atoms
in one cis-dppee ligand point to the same direction (Figure 4b),
while in each trans-dppee ligand of cluster 2, the separations of
the two P atoms and the two neighboring gold atoms are 4.45 and
5.61 Å, respectively. Compared to cluster 1, the separations of the
two P atoms of cluster 2 in one ligand become 0.85 Å larger, and
the lone pairs on the two P atoms in trans-dppee are disposed
toward opposite directions, which renders the retention of short
Au···Au contacts impossible. Therefore, triggered by photo-
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental details, X-ray structure determination, photo-
physical studies, and Figures S1−S12; crystallographic details
(CIF). This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*wwyam@hku.hk
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
V.W.-W.Y. acknowledges support from The University of Hong
Kong under the Strategic Research Theme on New Materials.
This work was supported by the University Grants Committee
Areas of Excellence Scheme (AoE/P-03/08), the ANR/RGC
Joint Research Scheme (A-HKU704/12), and a General
Research Fund (GRF) Grant from the Research Grants Council
of the Hong Kong Special Administrative Region, P. R. China
(HKU 7060/12P). L.-Y.Y. acknowledges the receipt of a
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Communication
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postgraduate studentship from The University of Hong Kong.
The authors would like to thank Dr. Eva Yi-Man Fung for mass
spectrometry measurements and Dr. Chi-Chiu Ko of City
University of Hong Kong for collection of one of the crystal data.
The Beijing Synchrotron Research Facility (BSRF) is also
acknowledged for providing beamline time of the synchrotron
radiation X-ray diffraction facilities.
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